1. The Field of the Invention
This present invention relates to optical communication systems. More articularly, embodiments of the present invention relate to routing channels of wavelength division multiplexed (WDM) optical communication systems.
2. Relevant Technology
Optical communication systems are becoming a substantial and fast-growing constituent of traditional communication networks. The use of optics in communication technologies is particularly desirable because of the immense potential bandwidth available for conveying information. The increase in bandwidth is due primarily to the higher frequency of optical signals compared to traditional wire or radio communication mediums. Thus, optics are especially suitable for use in such communications applications as telecommunications, cable television systems, local area networks (LANs), and the like.
Typically, optical communication systems use some form of waveguide, such as an optical fiber to transfer the information carrying light signal from one location to another, although some systems, such as satellite-to-satellite systems, may directly beam signals from one location to another. A waveguide is simply a device that confines and guides a propagating electromagnetic wave, such as light. An optical fiber typically includes a core section and a cladding section that allows efficient transmission of light over relatively long distances, such as tens or even hundreds or kilometers, depending on the strength of the optical signal and attenuation of the waveguide.
While the information carrying capacity of optical communication systems is high, it remains a primary objectivexe2x80x94as is the case with most communications technologiesxe2x80x94to increase the amount and rate at which information can be transferred over the communications medium, such as an optic fiber. There are however a number of factors that must be taken into account when an increase in bandwidth is desired. For example, it is not cost efficient to have a separate wire or waveguide for each individual signal. Nor is it reasonable to continuously add new channels to accommodate and provide additional bandwidth.
One solution to the bandwidth problem includes laying more fiber-optic cables to meet the increasing demand for bandwidth. Unfortunately, this approach is expensive and time consuming. Moreover, the approach is not always practical due to physical constraints.
Other approaches for increasing bandwidth involve utilizing the channel more efficiently. For example, one approach is to utilizes a technique called time-division multiplexing (xe2x80x9cTDMxe2x80x9d). This technique allows information to be conveyed from multiple sources on a single light signal. TDM consists of placing multiple signals in one wire or waveguide by separating portions of each signal by a certain amount of time. Each complete individual signal can then be recovered by sampling at the particular time slot allocated to that signal. Unfortunately, this technique does not sufficiently solve the bandwidth problem; certain interference effects and transfer speed limit the capacity of a TDM system to transfer multiple signals on the same wire or waveguide.
Another technique used for increasing the bandwidth of optical communication systems is referred to as wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d). This technique increases the capacity of an existing optic waveguide by using multiple wavelengths of a light signal to carry multiple signal channels. This technique can greatly increase the capacity and bandwidth of installed fiber optic networks. In practice, a WDM system typically employs multiple optical signals or channels from many sources. Each of these signals is assigned a particular channel wavelength band, or segment of the total spectrum. The multiple optical signal channels are then multiplexed, with a WDM multiplexer, to form single output optical signal, which can then be transmitted over a single waveguide.
It is desirable to demultiplex the optical signal into selected channels or groups of channels to route the channels to a variety of destinations. Exemplary WDM optical communication systems are described in U.S. Pat. Nos. 5,504,609; 5,532,864 and 5,557,442, the disclosures of which are incorporated herein by reference.
In many applications, such as optical LANs, cable television subscriber systems, and telecommunications networks, there is a need to selectively change the route of one or more channels of a multiplexed optical signal to different destinations. Such routing occurs when optical channels are sent to or withdrawn from an optical transmission line e.g., for sending optical channels between a terminal and an optical bus or routing long distance telecommunications traffic to individual cities.
However, this ability to provide xe2x80x9cwavelength selective routingxe2x80x9d, i.e., the ability to select and redirect one or more channels from a common WDM signal, has not been satisfactorily addressed. For example, existing solutions may first convert the signal from the optical wavelength domain to electrical signals, before they are selected/redirected. This process is not extremely efficient, and can introduce a bottleneck in the communications system. Moreover, repeated conversion of the optical signals to the electrical domain and back again can introduce signal error.
Also, selective routing of signals is not easily achieved, especially in a xe2x80x9cdensexe2x80x9d WDM (xe2x80x9cDWDMxe2x80x9d) communication system, where channels can have a center frequency separation of 100 GHz (or a wavelength separation of about 0.8 nm) or less. This is largely due to limitations in conventional optical switches, which generally switch all wavelengths (channels) at once. Under this approach, to selectively route an individual optical signal channel, the entire signal is demultiplexed by a demultiplexer into all the desired channels or sets of channels. After the signal is demultiplexed, optical switches are used to selectively direct the channel(s) toward its intended destination. To effectively utilize the total bandwidth of each route, the signals are typically then re-multiplexed after they are routed. As such, each potential route requires a separate demultiplexer, multiplexer, and optical switches for each optical channel. This placement of multiplexers and demultiplexers in series results in signal loss from a phenomena known as bandwidth narrowing. Additional signal losses (such as insertion losses, polarization mode dispersion, and ripple) from each of these components will accumulate, thus imposing cost penalties from required amplifiers to boost the signal and/or limits on the interconnection density in the network fabric. These sorts of inefficiencies are especially problematic in a DWDM network, and thus conventional WDM multiplexer and demultiplexer devices generally consume a large portion of a systems attenuation loss budget, and may suffer from additional performance deficiencies.
Therefore, and especially in connection with new and developing standards, optical routers and switches are needed that offer reliable and stable performance over a variety of environmental conditions. More particularly, it would be desirable to provide an optical switch that can select (i.e., drop or add) one or more optical channels. Preferably, such selection would be accomplished by wavelength demultiplexing/multiplexing only the selected optical channels, without affecting the nonselected optical channels. Also, it would be an advancement in the art to provide an optical switch that is small in size, is reliable, and which has only a single point of failure. In addition, the switch should have low insertion loss and adjacent channel tilt on all channels, and should be of a form that is easily interconnected or cascaded.
Given the foregoing problems and shortcomings in the prior art, it is an overall objective of the present invention to provide an optical device that provides the ability to switch selective wavelengths. A related objective is to provide an optical device that is capable of routing one or more selected channels in a given incoming wavelength multiplexed optical signal, while at the same time permiting the non-selected channels to pass without significant attenuation, signal loss, polarization mode dispersion, ripple, or bandwidth narrowing.
These and other objectives are provided by way of embodiments of the present invention, which is generally directed to an optical device that routes and selects one or more channels from a WDM signal and that does not significantly affect the unselected channels. In presently preferred embodiments, a wavelength selective optical router and switch selects one or more predetermined optical signal channels from a plurality of wavelength division multiplexed optical signal channels, and selectively routes the selected signal channel to one of a multiple number of xe2x80x9cdropxe2x80x9d ports. At the same time, all the other xe2x80x9cnon-selectedxe2x80x9d channels are reflected to a common port before passing through the switch. Moreover, the non-selected channels pass without significant attenuation or loss.
According to one embodiment of the invention, the wavelength selective optical switch has four ports, including one input port, an express port, an output port, and a DROP port. The optical switch receives an incoming signal comprised of several optical signal channels at the input port. One or more of these optical signal channels are then selected when passed through a wavelength selective filter integrated with an input xe2x80x9cflashlightxe2x80x9d that includes a ferrule for holding the optical fibers in relation to a collimator, such as a GRIN lens. The filter passes a channel or set of adjacent channels through the device and this portion of the input signal is considered xe2x80x9cdroppedxe2x80x9d from the remaining incoming optical signal channels.
The dropped channel can be switched to one of two selected ports with an optical routing element, such as a movable mirror. For example, in a first switch state, the xe2x80x9cdroppedxe2x80x9d channel can be transmitted unobstructed from the input filter to the output port (i.e. not dropped). In the second switch state, the mirror is moved into the optical path of the selected (dropped) channel to deflect the light beam towards the DROP port.
In a further embodiment, a common port, ADD port, and wavelength selective output filter are added. The wavelength selective output filter transmits the switched wavelengths and reflects a remaining portion (typically arriving at the common port in addition to any signals provided by the ADD port) to the output port.
The present invention solves or substantially mitigates the problems associated with combining conventional optical switches and wavelength division multiplexer and de-multiplexer devices, by providing higher performance, compact size and stable performance over many environmental conditions at reasonable costs.
These and other features, aspects, and advantages of present invention will become better understood with reference to the following drawings, description and appended claims.